An eddy current flaw detection is known as one of inspection methods for inspecting flaws in metallic materials. In flaw inspection for non-magnetic pipes composed of non-magnetic materials such as austenitic stainless steels, titanium, and copper alloys, eddy current flaw detection using an eddy current flaw detection probe of insertion type is widely adopted.
When flaw inspection for a magnetic pipe composed of a magnetic material such as a carbon steel, a ferritic stainless steel, and a duplex stainless steel containing two phases of a ferrite phase and an austenite phase is conducted by means of the eddy current flaw detection probe, used widely for inspection of non-magnetic pipes, accurate flaw detection is not achieved because an eddy current flows merely on the surface of the magnetic pipe and because a noise caused by local fluctuation in the magnetic permeability affects adversely the flaw detection performance.
Non-patent Document 1 discloses an eddy current flaw detection probe used for a flaw inspection of a magnetic pipe composed of a magnetic material of duplex stainless steel. In the eddy current flaw detection probe disclosed in Non-patent Document 1, a sensing coil is arranged around a center part of a pillar-shaped yoke and then permanent magnets whose magnetization directions are in the radial direction of the yoke and whose magnetic poles on the yoke side are different from each other are arranged around the yoke on both sides of the sensing coil.
Non-destructive Inspection, Volume 42, No. 9, 1993 (Heisei 5), pp. 520 to 526